Why does the author leave the narrative at Devon at this point and discuss the war as Gene later experienced it?

Remember that your are only allowed to ask ONE question - five seems to be taking it somewhat to an extreme, so I will focus on your first question, which refers to the last page of the novel where the action shifts from the younger Gene to the older Gene looking back at himself and reflecting on what he learned from the experience. 

To answer this question you need to focus on the specific narrative style that is used in the story. It is called first person retrospective narration, which means it is an older narrator looking back at his or her younger self. This is a very specialised form of narration used in other novels such as Great Expectations and Jane Eyre. What is interesting about it is that we as readers need to remember that alongside the youthful narrator explaining what is happening to him or her, we have the older, maturer narrator who also comments on the action, sometimes with disapproval.

What is fitting about this narrative departure at the end of this novel is that it reflects what Gene has learned from his experience and re-states the theme of the novel. Note how the older Gene does not share in the other boys' disillusionment, because he is able to reflect more deeply on what has happened. The older Gene reflects that the war is something far deeper and intrinsically a part of the human condition. The older Gene suggests that war emerges from a kind of ignorance deep within the human heart. It is this condition of ignorance that causes people to seek out an enemy and envision the world as a dangerous, hostile environment. The novel ends with the older Gene questioning the logic and worth of such an approach:

All of them, all except Phineas, constructed at infinite cost to themselves these Maginot Lines against this enemy they thought they saw across the frontier, this enemy who never attacked that way - if he ever attacked at all; if he was indeed the enemy.

Thus the end of the novel marks a narrative departure to give us this maturer ruminations - which would be unrealistic for the younger Gene to present.